One to One Atrioventricular Conduction via a Double Response with Two Levels of Block

Abstract

A 49-year-old woman presented with palpitations and progressive fatigue and dyspnea. An echocardiogram demonstrated normal left ventricular function and mitral annular calcification. Telemetry monitoring demonstrated an irregular tachycardia with periods of one to one and suspected 1 to 2 atrioventricular conduction (double response) as well as Figure 1. There are 3 mechanisms of block demonstrated among these 2 cycles. The first atrial depolarization is conducted through the fast pathway. The second atrial beat is conducted through the fast pathway with a normal HV and through the slow pathway with a longer HV. The third atrial depolarization is conducted slower through both the fast and slow pathway (longer AH intervals) with the slow pathway conduction blocked below the level of the His. The fourth atrial depolarization is blocked at the level of the AV node allowing for recovery of both AV nodal pathways and the infra-Hisian conduction system. The second cycle is very similar to the first, with conduction through the fast pathway, followed by conduction through the fast and slow pathways, but with a different level of block for the third atrial beat. The fast pathway conducts with additional decrement, and the slow pathway does not conduct from this atrial beat. The fourth atrial beat again blocks in the AV node. atrioventricular block. After analysis of her ECG, she underwent an electrophysiology study to provide definitive diagnosis and treatment. The recorded intracardiac electrograms are shown in Figure 1.With atrial depolarization, there were 2 patterns identified. The majority of the beats demonstrated either conduction down the fast pathway to the His and ventricle, followed by slow pathway conduction to the ventricle (double response), or conduction solely down the fast pathway. Intermittently, there are nonconducted atrial depolarizations with block in both the fast and slow pathways. This was initially misdiagnosed as atrial fibrillation, which has previously been described. 1,2 After the initial diagnosis was made, there were several other interesting findings. AV nodal block was the mechanism for most nonconducted atrial beats, except for intermittent infra-Hisian block (seen in Fig. 1) that could not be reproduced after ablation of the slow pathway. This